The mechanisms of cone bore growth have been investigated using experiments and modeling. In particular, stress-induced transformation of retained austenite to martensite, thermal-induced transformation of retained austenite to bainite, and low-temperature creep were experimentally investigated. Monotonic and cyclic tests on carburized 4320 steel with either 35% or 14% retained austenite content and on quenched and tempered 4320 steel were conducted. Transformation was detected by measuring the volume change of the specimens. The three ring model simulated bore growth during service and indicated the effect of the recondition treatment on the change in bore diameter. Both experiments and modeling indicated that cone bore growth could be caused by any of these mechanisms, depending on temperature, interference fit, residual stress, and heat treatment.